Trilogy and Trilian here. The classic bass in Kontakt does a brilliant job too.

And I still like my Broomstick bass from way back. Simple times but we were happy.

Of course there is always Bad Bassist from StrayWorx
 
 Turn the bad down to zero its its quite good.

I think it's a great time to revisit the topic and list all the goodies that have come along since 2012.
 
I'll add one more: Orange Tree Samples' Rickenbacker. I absolutely love that one. It sounds good in everything, not just Rush covers. Better have 8GB of RAM or better, though, because Greg sampled that thing down to the last detail. 24 round-robins, and each pickup sampled separately for that classic bright and mellow duality that Ricks are famous for.
 
And if you have full Kontakt but not much money, go grab one of Lyrical Distortion's $19 amped bass libraries. Not much variety, but they're instantly-usable classic bass sounds miked through an Ampeg like a classic bass guitar is supposed to be.

The bass in Cakewalk studio instruments is quite usable. If you got it with Sonar and have not tried it give it a go.

Agreed. SI-Bass isn't bad at all, except that the high notes are thin-sounding and sometimes the auto-glissando feature sometimes gets weird. But turn off slides or edit them back, stick to the lower octaves and add a little distortion and it's quite nice and very light on resources. 
 
Here's one that's a little more obscure...a patch in SampleLogic's Cinematic Guitars library called "P-Bass SVT". You might have Cinematic Guitars and not even noticed that it includes some basses. I'd used this one a year ago and forgot all about it until I listened to the tune recently and thought "that bass sounds pretty good, I wonder what I used...". That whole library has been a workhorse favorite.

If you like the bass here http://blog.cakewalk.com/guitar-month-bonus-pack-free-downloads/ there is a larger version in the Cakewalk store http://www.store.cakewalk.com/b2cus/ProductInfo.aspx?productid=10-CWGB1.00-20E which may go on sale in a week or so.

While I love Trilian, I often find its basses need quite a lot of work to sit well in the mix. A little too much low end in them. Although I guess that's better than not enough low end.

SI-Bass is sometimes just the sound you want. Sits in a mix very well too.

Some options for something other that Electric Bass Guitar or synth bass.
 
Boulder Sounds Guitarron I really like this one.
 
Ilya Eftimov - Contrabass Balalaika - another different sounding acoustic bass instrument.
 
Acoustic Samples AkousKontr - This is an upright bass for UVI workstation. I bought the Kontakt version before the discontinued it and switched to UVI, its the best upright I have. They also had an electric version but they do not seem to have ported that to UVI.
 
I have not found much in the way of acoustic bass guitars for Kontakt. There is a reasonable one in the Independence Pro factory library. In fact all the electric and acoustic basses are good, its the old yellowtools "Majestic" package, not full featured like Scarbee or Orange Tree but they sound nice. Unfortunatly apart from a couple of minor updates development of Independence seems to have stopped since Magix took over. It a shame because the factory library is better than both Kontakt's and Sample Tank 3 in many areas. (Sax, Bass and the Kirk Hunter orchestral stuff).