Lemme see.
You've got the Prinworks near The big CIS tower. Half a dozen pubs 'n' bars and a chinese buffet resturant. Not brilliant but always plenty of people about and you don't really any trouble. Full of CIS staff at a weekend too. Usually CallCentre Chicks

Sinclair's Oyster Bar in the Triangle and a couple of pubs down there (look for the big screen, next to the Selfridges/Marks & Spencers) Always hammered in summer when there's nice weather. Oldest building in Manchester incidentally.
Then there's Deansgate (one of the major roads through town). Plenty of pubs 'n' bars down there. Also next to Marks 'n' Sparks, opposite side of the Triangle. All the way down to the Town Hall if you fancy a crawl.
China town. There's a good karaoke bar opposite the CIS Portland St building. I don't normally go for karaoke but it's a laugh when your wasted.
Gay Village has some good bars and 1) you don't have to be gay 2) People don't assume your gay. Barbars is good and they have comedy unisex toilets.
That's all I can think of right now...