The Grados are nice, for sure, but I was never a huge fan of their sound. My SR60s lacked true bass, they sounded congested, and they were too grating on my ears with the upper mids and treble. They are very aggressive sounding headphones and actually not very competitive under $100 anymore (IMO).
If you don't mind a pair of larger over-the-ear headphones, I would completely recommend the Samson SR850 ($50), which are more or less a copy of the Superlux HD668B. They will be easily driven by a portable source, and they sound absolutely fantastic for the money. They are slightly on the forward, aggressive side, but they were actually pleasing to my ears compared to the Grados. They are semi-open and leak more sound than closed headphones as such. After 50-100 hours of burn in, I was floored by how good they sounded. I would not have been surprised if they cost $100-150.
Another option is the Panasonic RP-HTF600-S. For $30, they're really awesome headphones. They have a fun, somewhat bass-heavy sound signature. They're light, comfortable, and not very large. Unfortunately, build quality leaves a bit to be desired, and the drivers can get messed up pretty easily from air pressure (for example: pushing the headphones on your ears might cause pressure to push the driver cone into an unnatural position). They usually pop right back into place with a tap on the cups or if you apply a bit of suction to the drivers with your mouth. If you treat them carefully, they shouldn't give you any issues. The SR850, on the other hand, feels very sturdy.
I would also strongly recommend you look into the Creative Aurvana Live! headphones. Apparently they share the same driver as the now discontinued Denon D1001. I've heard they sound wonderful for the price as well.
Other options include the Audio Technica ATH-M50 and the Shure SRH440. The M50s are quite nice for the price if you can find them around or under $100. However, if you don't mind large headphones, the Samson SR850 will offer the best deal for under $100.
I would personally stay away from Grados if you want a more...accurate representation of your music or have sensitive ears. I would also stay away from the Sennheiser offerings if they're in the HD4xx line or lower. They're not so great for the money.
If you have any questions, feel free to PM me. I spend a good bit of time on the Head-Fi forums, and I'd recommend you check that place out for further suggestions.