kind of curious.
Republic bar is the main live music bar. Its on the main drag in North Hobart.
Further up Elizabeth street in north hobart is my personal favourite pub in town called the alley cat. Its got a really nice bar there and is probably more like a "Melbourne bar" than most others here. Also between those 2 pubs is the queens head, thats good if your 50.
The Brisbane plays a bit of live music but mostly punk/hardcore, you'll find that on the corner of Brisbane street and Argyle st.
Down in Salamance theres Irish Murpheys which plays cover bands over the weekend but during the week showcases a lot of local bands. Theres various other pubs scattered around the town which play cover bands as well.
The best venue in my oppinion is actually the unibar, but they only have live bands about once every 2 months or so, usually touring mainland bands. about 10 years ago
This all depends on what kind of music you're looking to see. If you're looking for the younger hipper crowd, you pretty much want to look into the Republic (www.republicbar.com) or the unibar. In my opinion the republic has a house party vibe, which can be great as long as the crowd is good (which, sometimes it's really not). So it can be hit-and-miss. The uni bar however has surprised me again and again, the crowds are always great and the atmosphere is surprisingly awesome. It's definitely my favorite venue in hobart.
If you just want to see noisy cover bands for free and drink beer then go to the victoria tavern or irish murphys, basically what russelleos said. The floor is sticky and the music hurts your face. There's also curlys which has a weird mix of live cover band and dj (alternating every hour I think), which can actually be kind of fun if you're in the mood.
For screamo / emo / metal there's the loft and the brisbane. I would not recommend the alley cat personally as I don't like the unwashed hippies it attracts occasionally. They tend to pull in the 'kooky' acts and solo artists if you're into that.
If you want to see old guys play old songs badly there's dr syntax. Or maybe this shut down...?
A lot of the touring deejays tend to play halo or syrup. Syrup is not a good venue at all, Halo can be decent depends on the night / act.
In Hobart, beggars can't be choosers, so the best thing to do is just check out what's on using a gig guide. One I use regularly is:
http://www.thedwarf.com.au/nd/whatson/tasmania about 10 years ago