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Have a look at Startcom. Free and the root is in most major browser/os's but there are some caveats such as having to re-register/renew every few months.
Tbh whilst I take the point about cost being an issue, for $10 just get a rapidssl from namecheap.com - job done.
Exactly, startssl.com worked great for me. Class 1 is free and easy to get. The lifetime of the cert is 1 or 2 years so that's not an issue. Your validation lasts only 30 days though, which means you'll have to prove your identity or ownership of your domain quite often when interacting with them. Doesn't mean the cert will expire after 30 days though! I second the namecheap.com recommendation though. Cheap enough.
I've been using StartCom for 3 years now and they rock. Can't go wrong with them and their certs being free for an year. Also, the owner of the company still runs most of it and detected recent attacks himself!
Namecheap.com was our initial low cost SSL supplier (US$ 10/year). Now bought a PositiveSSL cert from https://comodosslstore.com/ at US$ 6.50/year when buying for 5 years. The 5 years is beneficial to us. Now we only need to execute the renew process once every five years. Note: the optional free upgrade to EV is a separate cert with a validness of only 1 year (not 5).
I signed up with comodosslstore.com and got a certificate valid for 3 years for $7.50 per year (22.50 total). Previously, I had a cert from StartSSL, but they wouldn't renew it, alleging I wanted to defraud a similar-looking domain that was registered 10 years after mine. $8 per year is not worth the hassle with StartSSL!
From windows, using all browsers, the certificate used on their own website cacert.org/index.php?id=1 is untrusted. Google Chrome big red scary page. That doesn't look so good.
They are free and trustworthy, but their root certificate is not included in the 'trusted roots' lists of most browsers yet. This is sadly mostly a question of money. CAcert.org is working on passing an audit, the most important step to becoming included in browsers. Many Linux distributions already include the root certificate by default, but the huge Windows market is still out of reach. In any case, they're not 'trusted' yet in the way @asn187 asked.