Mxtabs [http://www.mxtabs.net]
A few riffs I made up and an exercie (Bb scale,Am Scale) and a few tips..

Kinda Funken bass riff..Play It kinda with the rythem of the Green Day
Song Longview..

G:----3-2-------5-3-2-----7-5-3-----3-2h3p2---|
D:--------3---------------------------------3-|
A:2h3-------2h3-------3h5-------2h3-----------|
E:--------------------------------------------|

Just somethin stupid...

(its a 10 not a 1-0)
G:-------9------5h7p5-------9------7h9p7--|
D:---10-7--5-5-5-----5--10-7--7-7-7-----7-|
A:-08--0--0-0-0-------08--0--0-0-0--------|
E:----------------------------------------|

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Good exercise..(Bb Scale) just do it over and over and faster and 
faster...

G:----------0-2-3-2-0-----------|Sometimes at the end I like to do
D:----0-1-3-----------3-1-0-----|a Octave Jump so after you hit the
A:1-3-----------------------3-1-|last 1(Bb) jump to [G:-3-:] and back
E:------------------------------|down to the 1(Bb)

Am Scale..
G:-------------0-2-2-0-------------|
D:-------0-2-3---------3-2-0-------|
A:-0-2-3---------------------3-2-0-|
E:---------------------------------|

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The one thing you hear at baseball games and stuff called Go team go

G:------------------------|333----|
D:--0-1-23--0-1-23--0-1-23|-------|
A:-1-------1------ 1------|---111-|
E:------------------------|-------|

You can end many differnt ways there are 3 that I like and they are just
Octave jumps

like the one I showed on there
G:333----|
D:-------|
A:---111-|
E:-------|

Other
G:333-3--|
D:-------|
A:---1-1-|
E:-------|

Last one
G:333-3-3-|
D:--------|
A:-----1--|
E:--------|
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Tips

Now Ive been playing for a while Played all Kinds of differnt music.
The best I can find to have good bass riffs give you a good challange
and have made me better are songs by these artist...

Thrice-Almost any song of there\\'s will give you a good workout..examples
Stare at the sun (as of now still the hardest song I know) ,Blood Clots 
and Black Holes,To what end etc.

AFI-Many of their songs are good..

RHCP(Red Hot Chili Peppers)-Almost any song is good..

Avenged Sevenfold-Most songs the bass plays same as guitar but it creates
a challange...

Thats just a few that I think you should listen to and you will get better
as you learn some of their songs..