You can find a list here:

Some of the more popular / common ones are

• Orthodox Union: the letter “O" with a “U" in it.
• Chicago Rabbinical Council: represented with “cRc" inside of a triangle
• Kof-K: the Hebrew letter כ (which is the first letter of כשר —kasher, meaning “kosher” with the letter “k" inside of it

Other certifying agencies and their associated trademarks can be found at the link above

Note that the use of these trademarks for certifying food as kosher is highly dependent on US trademark law and the fact that, by virtue of such laws, these agencies can ensure that only foods that have actually been certified by those agencies have their trademarks. In other countries, this doesn't necessarily translate; there are other countries in which one has to visit a kosher certifying agency's website to look up a list of brands/items that the agency has certified, rather than using these symbols to identify kosher items.