For fret rattle, I like the tube saturation PC Plug In... you know... the big nob.
 
I split the track so that the higher sounds go to a bus and put the tube saturation on the bus.
 
Like the whole use two copies of a multi band and solo the low bands on one bus and the high bands on the other bus. Then put the tube saturation on the higher bus.
 
If you turn it up, you can get a nice crunch from the fret noise.

i might take a look at the dimension pro bass patches because several of them have string snaps built into them that you could layer at certain points. try full slap dark and throw a 4 pole low pass filter on it with some resonance then upping the shift parameter and putting a fast flange on it which can make it sound less robotic.  
open the prv and drag up on the piano keys until you start hearing the snaps. 
you can also take a look at some of the sfz files in the dimension pro multisamples library.  special sample types are notated like this
<region> sample=Fretless Bass\\Jbfless1_sld_up_38.flac key=e6
<region> sample=Fretless Bass\\Jbfless1_X_03.flac key=f6
sld is a string slide and X is a string snap.  
 

WOW!! thank you all for these suggestions!
 
I haven't had a chance to read all of them yet, but what i've seen so far are a lot of awesome suggestions to bring out attack transients etc. What I originally meant was I was wondering if there was a library that had that particular effect in it. I also forgot to mention that I have the Scsrbee Jazz bass. Its just that, like most libraries, it wasn't played/recorded that aggressively.
 
I had hoped that someone knew of a library that was sampled with that particular aggressive technique.
 
Anyway its gonna take a couple days to work through all of these suggestions. Thank you all for the detailed informative responses.

When you ask NASA how to build a rocket... giggle

HI Bruckner2 Steve Harris comes to Malta for Holiday very often,what a nice person  he is,if you like next time I see him, I`ll ask him if he can come and play it for you.Dont be surprised he say yes,but I`m only joking ,I`m too shy to ask him that.
The best thing is to get a bass player and record it,he dont have to be Steve Harris.
I think it`s qiucker.
Peace.

maltastudio...Totally off topic, but I have to get my friend from Malta to send me some Twistees again.
 
Back on topic.  I agree that the quickest and most natural sounding way is to have a bass player play over top of your  synth bass track.  You can filter the two tracks so that the click part comes mostly from the live bass with the synth bass providing most of the sound.  You would not need a particularly good bass player or instrument.  Just one that slap in time to your track.

Leadfoot

KPerry
You only seem to get this effect with Fender basses

Any bass will give you that sound. It just depends on your technique and the aggressiveness with which you play. Just ask my basses.

OK, in my experience, you only get this with Fender basses (and I've had bassists try to replicate with other makes) :-)  I don't know what it is...it could even be down to subtlies in how the fingers end up because of the bass shape, but it's definitely a difference.

bruckner2
thanks KPerry. I'm curious why or what you do like about his sound. I saw Maiden on their first tour at Long Beach Arena in 1981 and was blown away by their tightness and sound. I've been hooked since, although their latest studio album seemed kinda pieced together in spots, a bit chaotic?? For me 'Somewhere in Time' and 'Live After Death' are their best.  I'm also a big Geddy Lee fan! The thing about Steve Harris' "clicks" that I like is the way it interacts with the drums while at same time punching up the low end under the guitars. I don't know if that makes sense. For me describing music is like talking about colors  

Maiden are one of the few bands I really want to see that I've never been able to...different reason every time (I was able to tick off Dave Gilmour and Roger Waters and Rush within about 6 months of each other, followed by Jethro Tull last year :-)), so don't get me wrong: I love Maiden as a band, I just find Steve Harris' bass sound irritating a lot of the time.  Not always, but I can find extended listening annoying.
 
I think Geddy's bass sound has got worse recently too - I think his Rickenbacker days were his best sound-wise.  I love that piano-like Ricky sound, but how it can also growl so much, especially when you play it like Lemmy!

KPerry
I think his Rickenbacker days were his best sound-wise.  I love that piano-like Ricky sound, but how it can also growl so much, especially when you play it like Lemmy!

I agree. Listening to the bass line of Red Barchetta in my head right now. To me, Moving Pictures was Geddy's peak.

I use Broomstick Bass for nearly all my bass needs and it has a great set of patches that click as well as has articulations for clicks. 
 
Also, I thought the Cake studio instruments had a pretty clicky bass patch too. The SI series is surprisingly good, I use it for sketching and sometimes don't even replace the sounds.
 
Of course you could always drop some coin and get Trillian, which is sure to have what you need.