I love tabs, but lets face it, they are very rarely 100% correct. I know that the TASCAM MP-BT1 Portable MP3 Bass Trainer (see link below) has a feature that lets you listen to the bass track only. Is there anyway to do this myself on a computer? There are some songs I want to transcribe, but I am having trouble hearing the line.

Also is there any site that has bass tracks singled out?

TASCAM MP-BT1 Portable MP3 Bass Trainer
i have found searching youtube can offer decent results but google is worth a try.
What bass tracks do you need singling out?

There are many songs that have been split apart on youtube, these are songs from Rock Band or Guitar Hero that have master tracks.

For example, there's some Muse, Metallica, bit of Megadeth, Pearl Jam, etc.
There are mixing programs you can get a hold off that let you take out parts of songs, or at the very list make them much more quiet, im not sure if Logic Pro does it or not.
Every master was once a begginer.
With a music sequencing program you can't do anything like that, I'm afraid.
You can boost the bass frequencies and lower the rest to make it more prominent in the mix, but you can't single it out altogether unless you imported all the tracks separately.

I'm fairly certain there is a website that has alot of famous songs pre-mastered and multi tracked.
In college we looked at Bee gees, James Brown and Christina Aquilara songs this way.
So there is definatly something out there that could do it for you, have a good hard look.