www.xanproxy.be - no ads except the first page.
ieproxy.com - no ads.
www.techtakeover.com/search - no ads.
www.spysurfing.com - no ads except front page.
www.myfreeproxy.be - ads on the front page
worldwideproxy.com - no ads.
www.proxytastic.com - contains ads.
www.nomorelimits.net - contains ads.
www.proxifree.com - contains ads.
browseatwork.com - contains ads, but clean looking
www.netsack.net - clean looking but with popup ads.
www.afreeproxy.com - contains ads.
cgi-proxy.net - contains ads on the first page, slow.
www.coveredtracks.com/homep... - no ads. but rugged site.
www.easyproxy.org - no ads except the first page.
www.privatizer.net - contains ads on the first page.
www.secure-tunnel.com - no ads.
www.spiderproxy.com - no ads.
thestrongestlinks.com - contains ads on the first page, slow.
www.unipeak.com - slow, no ads.
betaproxy.com - contains ads.
proxy7.com - contains ads.
webwarper.net - contains ads.
www.chronicpulse.net - contains ads.
www.space.net.au/~thomas/qu... - multiple proxys
www.proxyking.net - contains ads.
w3privacy.com - contains ads.
freeproxy.us - contains ads.
www.goproxing.com - contains ads.
greenrabbit.org - contains ads but small ads
hujiko.com - contains ads.
poxy.us.to - no ads.
hidemyass.com - no ads except front page.
proxify.com - contains ads.
projectbypass.com - contains ads and popups.
www.proxymouse.com - contains huge ads.
letsproxy.com - contains ads.
www.the-cloak.com/login.html - no ads.

Update:
Thanks to the anonymous poster.

Secure pages and with no ads:

www.stday.com/cgi-bin/nph-p...
www1.ft888.net/cgi-bin/nph-...
www2.ft888.net/cgi-bin/nph-...
www3.ft888.net/cgi-bin/nph-...
www.secure-tunnel.com/SafeB... (also available via http://)
www.dongtaiwang.com/cgi-bin...
www1.dongtaiwang.com/cgi-bi...
www2.dongtaiwang.com/cgi-bi...
www3.dongtaiwang.com/cgi-bi...
towel.greyfyre.info/proxy/i...


Insecure pages (mostly no ads):

pbkill.com/scgi-bin/nph-pro...
darwin.servehttp.com/cgi-bi...
www.brainflux.org/cgi-bin/n...
starkravingmad.org/nph-prox...
barracuda.tx.envision7.com/...
www.q2r.net/pc/nph-proxy.cgi
www.e-ronin.com/cgi-bin/sha...
www.groothuijsen.nl/cgiprox...
www.vtunnel.com/nph-proxy.pl - with a little ad but still ok.
70.84.193.226/nph-86prox.pl
www.neoambient.com/86prox
www.ipent.com/ihfc/nwochat/... - contains ads, encodes url.
www.q2r.net/pc/nph-proxy.cgi


PHProxies:

www.arnit.net/utilities/web... - contains ads on the first page.
www.browseany.com - contains ads.
www.zhaodaola.org/web - contains ads.


No Input box by default:

www.gouc.fr/cgi-bin/nph-pro... - no ads, french language.
heshan18.com/cgi-bin/nph-pr... - no ads.


No images:

www.floon.com/cgiproxy/nph-...


Other pages with ads:

www.netshaq.com/cgiproxy/np... - no input box by default
www.proxy-surf.com/nph-prox...
www.bywhat.com - lots of ads.
www.fireprox.com/nph-proxy.pl - same as above.
www.nethider.com/cgi-bin/np... - popups, ads taking half of the browser space.
www.safesurfers.net
www.usbm.de/cgi-bin/userbea... - popup, ads, no images.
proxy.perlproxy.com/p
proxy.cecid.com/p

Quick update:
www.twobeep.com - no ads.

Update
Thanks to alex-and-r, RandomC, and an anonymous poster

www.guardster.com - contains ads.
concealme.com - contains ads.
www.twobeep.com - no ads.

Update:

anonymizer.nntime.com
radiofarda.be

Update:
thanks hybridstorm

ibypass.org - contains ads. (also available as .com .net .tk)