1. Matrix Cube Headphone Amp: This headphone amp comes with a DAC, which is a great deal at $300. This makes it almost a complete package when you want to listen to some good music on your decent pair of headphones.
2. HiFiMan EF-5: At about twice the price of the EF-2A, this amp is supposed to even out sound from harsher-sounding headphones. It adds a bit of body to the sound, but takes a while to warm up when first switched on.
3. HiFiMAN EF-2A: This affordable headphone amp doesn't play nice with all types of headphones, but the sound sure is nice when you find a pair that does work, like Grados.
4. Firestone Audio Cute Curve: The MSRP of this amp is $199, but it can be had for cheaper online. It's a low distortion amp that will preserve the sound quality of the source of your music. It's got a high current output, so it delivers powerful sound.
5. FIIO E9i Desktop Headphone Amp: This amp works for your iPhone and iPod, and it's an interesting way to listen to your mobile device with a decent pair of cans without investing into one of those headphone amp cases that will become obsolete once Apple releases a new iPhone. If the iPhone comes with a new dock connector, you may want to wait before getting this.
(Images: as linked above)