Gurren Lagann is a poor mans Getter. Just skip it and watch/read the many installments of Getter Robo.
Also seek out:
Texhnolyze: cyberpunk show that isn't grimdark, it's somewhere in the lightless valleys on the other side of grimdark.
Dennou Coil: slice-of-life cyberpunk involving kids. It's awesome.
Eve no Jikan: show about robots and human-robot interactions. I dare say it would make Asimov himself proud.
Black Lagoon: un-ironic homage to 80's action films. Frankly it's better than most of those films.
FLAG: real-robot show set in fictitious war-torn country. Basically it's all about UN peacekeeping operations.
Tokyo Magnitude 8.0: massive earthquake hits Tokyo (no, really? I know, you're shocked), brother and sister struggle to survive. Focuses more on the mundane issues of survival that such a situation would lead to than action movie nonsense.
Seirei no Moribito: vastly superior adaption of what was already a pretty damn good book. For my money one of the most refreshingly original entries in the fantasy genre of the last, oh, 15 years at least.
Dai Mahou Touge: dark parody of the magical girl genre. Short and sweet, some hilarious stuff in here.
Galaxy Express 999: only seek it out if you're willing to be in for the long-haul. 100+ episodes, 3 movies, plus a couple manga. I think it's Leiji Matsumoto's greatest creation (and that really is saying a lot). Calling it an explosion of pure creativity would be a massive understatement. Any description would probably sound retarded, but trust me, it's amazing.
Legend of the Galactic Heroes: already been mentioned, but it should be mentioned again. The finest space opera ever made. Period. 110 episode OVA adaption of a series of books by Yoshiki Tanaka, plus 3 movies, plus 52 episodes of simply prequel material. There's a manga too, but good luck finding any translations of it. The actual technological aspects of it aren't that great, I'll admit. Frankly you could pretty easily strip out all the space parts of it, make the ships surface based and set the whole story on a water heavy planet and it would still be more or less the same. But the author was someone who grasped that the most important parts of a story are plot and characters, and chose to focus on them rather than drowning everything in hyper-detailed fancy space tech. Both factions get amble coverage, both have large, likable casts (and a few assholes), and both have valid points. Neither side is the good guy, nor the villain. They're just people. All the characters are great, but the Alliances Yang Wen-li rises above all the rest for most fans, myself included. When you see it, you'll understand why.
Oh, it also has the greatest soundtrack of anything ever:
http://www.youtube.com/watch?v=upS-pi-sOAQSeikai no Senki/Crest of the Stars/Banner of the Stars/Battleflag of the Stars (lolnames): often thought of as a lesser sibling to LoGH, in many ways I think it's the complete equal of that franchise, and in some ways (especially the battles) it's actually superior. Grand space opera, but with a primary focus on the two fairly minor characters and their relationship. Extremely good character development. Outside of the two mains you have a fairly large recurring cast featuring several truly unique characters, including one woman who can only be described as an internet troll in an admirals body. The battles are pure sex and it's basically the last anime (that I know of anyway) where big battles were entirely hand-animated, so no terrible CGI in this franchise. Well, not quite...three full 'seasons' were made (Crest of the Stars, Banner of the Stars, Banner of the Stars II) covering the first 3 books (oh, right, it's also based on a series of novels, this time by Hiroyuki Morioka), then they made a 2 part OVA that was far too compressed and featured horrible CGI. Worth seeing I guess, it has some nice bits. Book series is continuing on, but no further animated adaptions in sight. Some fans are slowly translating the books though.
There are many more things I could list, but I don't want to make too large a post. I can provide download links for any of the above shows if anyone is interested.