I could be lazy like Sangaz and just post my list since he and I like a lot of the same shows but I'm just gonna give you a list.
If you're looking for something airing this season then there's only three I can really recommend.
1. K-On! - Best show of this season. Lovable characters, enticing story, great music. This show has it all. Another KyoAni great.
2. Fullmetal Alchemist: Brotherhood - If you've read the FMA manga you'll know how this goes. The animation is cleaned up and more crisp and I like the new VAs for the characters. Watch it yourself and see.
3. Valkyria Chronicles - Based off an amazing PS3 game. Takes place in a fictional wartime 1930s Europe. The first episode showed a lot of promise and it's worth looking in to.
And now for my super long list of series that have been completed.
Tengen Toppa Gurren Lagann - An anime that I will not think twice about calling the greatest series of all time. Made by the masterminds at Gainax it's what happens when Evangelion and FLCL merge. With a diverse cast of characters, hilarious moments, unique mech designs and (literally) universe changing battles this show will blow you away.
Gundam 00 - You said you liked Gundam. While kinda spiraling out of control near the end of Season 2 with a couple deus ex machina moments I still found Gundam 00 a joy to watch from beginning to end. A movie will be released in 2010 to finish up the story and the series just ended so now's a great time to catch up.
Sayonara Zetsubou Sensei - This is the funniest show I've seen in years. Characters are purposely stereotyped and it has a ton of satire in it. Definitely worth a watch if you want to laugh.
Clannad - A touching love story rounded out by comedy this show balances itself in every way. There'll be times when you're laughing your ass off and times where you'll be on the verge of tears. A great story that'll get you attached to characters like never before.
Maria+Holic - A lesbian goes to an all girls school to find her soulmate but ends up falling in love with an abusive cross dressing guy. I think that's all I need to say.
Welcome to the NHK - A show that takes a deep look into the lifestyle of a recovering hikkikomori. Looks at a ton of social problemsand disorders and has hilarity dispersed at just the right times.
Toradora! - A nice romantic comedy this show was enjoyable throughout its long run. The female lead Taiga may seem violent in the first episode but don't let that turn you off from this series.
Phew.....I'm just gonna stop there.