Tetraodon Biocellatus, the figure 8 puffer.
Perfect size tank for just one fish.
Ideal if you have other tanks, even better if they have some snails in. Free puffer food!
Tetraodon Biocellatus "Diddy P" | Flickr - Photo Sharing!
My F8 puffer in a similar sized tank, eating snails.
Figure 8 Puffer Profile
Excellent page on puffer care.