I help to maintain a map repository at
http://mercenariesguild.net/base/LOTTSA, LOTTSA maps there, and a +300MB map pack (start the download overnight

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I'll give you the maps I'd recommend and my worthless little blurb about them:
Hamanuptra - SST plays this a lot. It's good on large servers. Don't complain about it, it's a port so it wasn't originally built for tremulous. Even so it can play very well though it tends to stall mid game due to the fact that the humans always go topside and the aliens always go to the dead end on the top floor. On rare occasions when they don't it is usually a slaughter. Humans pin aliens in their base, aliens gain s3, aliens pin humans in their base, timeout or aliens win. I've only played it on SST though, so the matches will probably be more sensible on servers with a reasonable amount of BP (You won't spend the last 3rd of a game fighting a base

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Ancient Remains - If you don't have it already, you will need it at some point. Beautiful map. Alien sided unless the humans know the map well. Don't let people lie to you though, I've seen good humans take that map. Oh, and no matter what anyone tells you, don't let humans build in the lava room. It's a campfest where they are pinned in with little to no hope of escape. Good aliens will even build them in

UTCSb2 - thrown in lots of places. A small symetrical map that isn't like atcs, but you'll think of it due to the maps small size. Fun map with good teams, short map with feeder teams

atcszalpha - Thrown in lots of places. ATCS if it was a big map I guess. It has ATCS in the name so people love it. Don't get me wrong, I've had lots of fun on this map and don't dislike it, but it is mostly a camping map and I don't fool myself, people vote on it cause it has atcs in the name.
procyon - Thrown in lots of places, beautiful and huge, you may want to
load this one up locally and get to know it or you will be completely lost

Take note of the signs too, they will help.
pulse-beta3 - this map seems to have become less popular, but was VERY popular for a while and probably will be again. It is very large and unique. If it's not the most beautiful map in terms of detail, then it is close. If you don't wander through this one locally for a while, you'll be finding tunnels and nooks and cranny's for months

thermal - SST used to play beta 2 of this map a lot. Not sure anymore. Still, it is a nice map and can make for some fun games. The downside is the lamer spot for humans is too easy to move to early in the game, so the potential for a campfest goes up quite a bit.
derelictb-beta04 - Some people really don't like this map, I am not one of them. The downside I see to it is the difficulty of moving the human base outside of it's default room. Still, I've played lots of great matches here. Plus, I like yellow

Flap - Brand spankin new, so people are checking it out a lot (as they are with ancient remains). First use of jump pads I've seen in tremulous and they work very well. A beautiful map, I suspect that it may have a fatal flaw in that aliens can't evolve in their base, though that may balance out the advantage aliens get at the jump pads. TBO, I haven't played this one enough to know but, the games I have played have been pulse pounding and fast.