One of the best strategies to gain backlinks is to bookmark your website in dofollow social bookmarking sites. Back links like these will help you get tons of free traffic to your websites.

Here you have a list of such sites (with their pagerank).
Accurate as of Dec 1st 2008

1 – http://slashdot.org (PR9)
2 – http://digg.com (PR8)
3 – http://technorati.com (PR8) (Only Blogs)
4 – http://www.furl.net (PR7)
5 – http://www.backflip.com (PR7)
6 – http://www.hugg.com (PR7)
7 – http://www.mixx.com (PR7)
8 – http://ma.gnolia.com (PR7)
9 – http://www.connotea.org (PR7)
10 – http://mystuff.ask.com (PR7)
11 – http://www.reddit.com (PR7)
12 – http://www.dzone.com (PR7)
13 – http://www.folkd.com (PR7)
14 – http://multiply.com (PR7)
15 – http://www.searchles.com (PR6)
16 – http://de.lirio.us (PR6)
17 – http://www.dotnetkicks.com (PR6)
18 – http://www.bloghop.com (PR6)
19 – http://www.plime.com (PR6)
20 – http://www.bibsonomy.org (PR6)
21 – http://www.clipclip.org (PR6)
22 – http://linkagogo.com (PR6)
23 – http://www.spurl.net (PR6)
24 – http://www.zlitt.com (PR6)
25 – http://www.indianpad.com (PR6)
26 – http://www.tumblr.com (PR6)
27 – http://www.myvmarks.com (PR5)
28 – http://www.listible.com (PR5)
29 – http://www.bringr.com (PR5)
30 – http://faves.com (PR5)
31 – http://www.linkinn.com (PR5)
32 – http://spotback.com (PR5)
33 – http://www.mylinkvault.com (PR5)
34 – http://my.xilinus.com (PR5)
35 – http://linkatopia.com (PR5)
36 – http://www.bumpzee.com (PR5)
37 – http://www.bookmarktracker.com (PR5)
38 – http://www.healthranker.com (PR5) (Only for health retated websites)
39 – http://www.a1-webmarks.com (PR4)
40 – http://www.wirefan.com (PR4)
41 – http://www.bmaccess.net (PR4)
42 – http://blogmarks.net (PR4)
43 – http://www.diigo.com (PR4)
44 – http://www.oyax.com (PR4)
45 – http://socialogs.com (PR4)
46 – http://www.contentpop.com (PR3)
47 – http://www.pixelmo.com (PR3)
48 – http://www.memfrag.com (PR3)
49 – http://getigadget.com (PR3)
50 – http://www.jumptags.com (PR3)
51 – http://www.plugim.com (PR3)
52 – http://www.syncone.net (PR3)
53 – http://postonfire.com (PR3)
54 – http://www.business-planet.net (PR3)
55 – http://www.hatedorloved.com (PR3)
56 – http://www.bookmarks.com (PR3)
57 – http://www.yattle.com (PR2)
58 – http://www.kapely.net (PR2)
59 – http://www.givealink.org (PR2)
60 – http://totagit.com (PR2)
61 – http://socialblink.com (PR0)
62 – http://wigleytagz.com (PR0)
63 – http://buzztagz.com (PR0)
64 – http://youtagz.com (PR0)
65 – http://space-ed.com (PR0)