I've searched and searched and can only find one 2.5 inch HDD that is larger than 1TB (Western Digital has one model). 3.5 inch HDDs have kept growing to even 10TB, but 2.5 inch drives seem to have hit a brick wall. Are there any 2.5 inch HDDs larger than 2TB?
If you can use an external drive, there's the Toshiba 3TB Canvio HDTB330EK3CA (catchy!) It's 3TB and 2.5 inch, but it's expensive. Expensive to the extent that you could get 2 2TB drives for less than the 3TB. If it's just space and size you're looking at, this could suffice - otherwise, perhaps not.
If it's an internal drive you want, Toshiba also have the MQ03ABB300, which is a 2.5 inch 3TB drive for internal use. Again, it's got the storage and the size, but it carries a premium price.
My recommendation? Wait until these are cheaper. Meanwhile, get yourself a 3.5 inch drive, or 2 2.5 inch drives.
Just to update this with new products: 4TB 5400RPM Seagate 2.5"
All for $193.04
This drive offers the highest capacity for the form factor so far. While not as fast as a 7200RPM drive, if you are looking for mass storage, this will do well. Consider a small external enclosure with multiples of these for a media server.
As the original question does not state what purpose the drive is needed for, I feel this drive would be great for general use.
Seagate has recently released a 5TB drive for $85. It's also 5400RPM, but that doesn't matter much these days, because if you're very performance-sensitive, SSD is the way to go anyway.