Other than the usual ones mentioned here, in a rails specific environment, I use rsense[1], acp[2], rails.vim[3]. This works really well for me and I would recommend it to any rails developer.

[1] http://cx4a.org/software/rsense/

[2] http://www.vim.org/scripts/script.php?script_id=1879

[3] http://www.vim.org/scripts/script.php?script_id=1567