There are two guilds which are most important here: Assassins and beggars. These are dealt with separately below, as are bards. All the other guilds are dealt with in groups, below.
Craft Guilds[]
These are the myriad guilds of gemcutters, barbers and dentists, butchers and bakers, and so on. Thieves will not have dealings of note with many of them, but there are one or two exceptions. Pawnbrokers are an obvious possible "front" for thieves and fences, likewise moneylenders (with their safes and secure chests and the like). Thieves may thus cooperate with many such, and minimize robberies of the rest! Guilds which take young apprentices may be paid small sums for handing over to the thieves youngsters who seem to be highly dexterous and capable of developing into thieves, given suitable training. In special instances (e.g., thieves allied with seafaring smugglers), other alliances (e.g., with the seamen's guild) may be of note.
If there are unusual thief-craftsmen alliances (or oppositions), the DM should script them individually; they will need individual rationales, which should be explained and noted!
Adventuring Guilds[]
In some cases there may be an unusual reason why thieves are strongly opposed to (or allied with) an adventuring guild (including temples, etc.) in a city. The most notable example is a cordial alliance with clerics of a deity favoring thieves (e.g., Erevan Ilesere for half-elven and elven thieves, or Olidamarra, in Oerth; or Mask in Faerun). If some particular type of multi-class thief is common in a city for some reason, then there will obviously tend to be a stronger link between the two relevant guilds than usual. For example, if mage-thieves are common, then the Guild of Wizardry will take a definite interest in the activities of thieves. Under such conditions, the mages will probably not attract unwanted attention from light-fingered thieves (although few sane thieves try stealing from mages anyway).
But other possibilities exist. Consider a burgeoning frontier town, which is close to wild hills and forests with bountiful resources. Furs, gold from prospectors, meat from hunted animals, even some gems from a small mine; all these and more pour into the town, which grows rich and attracts many new settlers. Unfortunately, it attracts humanoids and bandits (as external threats) and many thugs and foreign cut-throats and evil thieves (as opposed to the neutral-aligned indigenous thieves). The rulers of the town grow fearful . . .
A twin alliance springs up to defend the town by stealth and cunning. Rangers patrol the distant countryside to give advance warning of marauding humanoids or bandits. Within the city, the thieves use their skills to tip off the powers-that-be about unsavory types arriving from outside, conspiring robbers and thugs, and the like (and may deal with a few of them themselves). Bandits beyond the town have spies inside it; the thieves tip the rangers off about this, and pass on intercepted messages. The thieves trade a magical shield (which they cannot use) they got from one of the thugs they dealt with to the rangers (who can use it). The rangers hand over a magical shortsword they took from a hobgoblin leader in return. Half-elven thieves and rangers share a drink together in an elven tavern, sharing the latest dwarf jokes. The two groups then save money by a joint bulk purchase of leather armor, and so it goes on . . .
Having an unusual, unexpected alliance like this spices up any adventure locale. It's well worth a DM's time to devise such a backdrop, to make a town or city unusual and particularly memorable.
Thieves and Assassins[]
Assassins are not a separate character class (in AD&D® 2nd Edition rules). They are simply people who are awfully good at killing other people. But the skills of the thief are valuable for this, above the natural skills of any other character class, because they are skills of stealth and sneaking about undetected. Many assassins will be thieves, multi-class thieves, or characters who began their adventuring life as thieves but then changed to another character class.
The assassin kit earlier in this volume adds detail to this natural affinity. Going further, the multi-class thief is probably the strongest option for assassins. Combining the skills of the thief with spells such as invisibility, jump, knock, levitate, rope trick, and sleep, just to consider a few low-level spells, makes for a potent assassin indeed. The fighter/mage/thief has all these advantages, of course, plus the bonus of additional hit points, and is thus an especially attractive option, although progress is slow.
So, given that many assassins are likely to be thieves, there will always be a fairly strong link between the two guilds. Cool relationships are possible if the thieves have "gone respectable" (become merchants themselves, etc.), or if the assassins are especially evil, ruthless sorts under a truly heinous guildmaster. Not unusually, though, the two will have at least moderately friendly relations. Information is quite likely to be exchanged between the two guilds, the guild seniors may meet occasionally to fraternize and discuss prospects, thieves may be paid to do spying groundwork for an assassin setting up a kill, apprentices of one guild who may be suited better to the tasks of the other may be exchanged; there are many ways the guilds can cooperate.
Thieves and Beggars[]
Beggars can be excellent spies. Their presence anywhere outside of the higher-class areas of cities and towns will go unmentioned, if they are seen at all. Beggars ply their trade everywhere, and some of them may just be sleeping off the effects of drink. If one is hunched up against packing boxes by a warehouse, so what? Anyway, no one wants to get too close to beggars. They have an unpleasant range of startlingly contagious diseases, and approaching them is an open invitation to lice to infest your person—just for starters. And, of course, beggars are harmless (in the sense that they are too feeble, dim-witted, drunk, etc., to cause anyone any harm). For all these reasons, well-trained beggars are a real boon to the enterprising thief. They can get into places where other people would look suspicious, they often go unnoticed, and they don't have a lot to do apart from keep their eyes peeled. Just the people to have hanging around the places a thief intends to rob. After the job, the grateful beggars can be given a little percentage of the take.
Beggary can even be semi-professional, in large cities in particular. Dominant beggars control lucrative patches of territory, where they know that pickings are richest. Fit and able-bodied people, their senses undulled by illness or drink, can pose as beggars for a better living (in a rich city) than they can make by honest means (e.g., as farm laborers). Such people would make excellent spies. A Guild of Beggars is by no means unlikely in many city settings. And, since children make very appealing beggars as they look soulfully up from their sad little eyes and beg for a penny for a poor orphan, such a guild will find and train such rapscallions. The more talented (dexterous and/or intelligent) ones may well be sold on to the thieves' guild, for a consideration, to become apprentice thieves.
Thieves and Bards[]
Although thieves and bards are both rogues, they have no need of extensive contacts. While bardic colleges train bards in some rogue skills which thieves also possess, this is really to be seen as part of the jack-of-all-trades range of talents the bard possesses. Bards don't think like thieves, and they don't behave like them. Perhaps the closest these two groups come to direct dealings is when a neutral evil bard becomes an assassin and the assassins' guild has good relations with the thieves' guild!