If you liked Ecco... then you will love
Aquaria.
The Demo is free .. and its huge.
I spent most of yesterday playing this.. the graphics are beautiful. The more you play it the more you realise it has to offer... At first it may seem a bit "floaty" as you go through the training stages, but later on when you get special powers it becomes a lot more tactical.. and then would appeal to those that like the "shoot to kill", as you can change the character in between the floaty mermaid who searches for items and completes puzzles (more appealing to girls) to the power charged Naija who fights the under-water baddies (more for the guys).
Its a long time since Ive enjoyed a game like this. A friend came round whilst I was playing it last night and was also impressed with the game play and ended up buying it out-right.
Warning - because the demo is so big - you could find yourself playing for days on that alone.
I believe it was only officially released on 7th Of Dec.. yet already it has quite a following and has won the IGF award for 2007.