Faendal

Faendal is a local archer of Riverwood. He and Sven have a rivalry between each other for Camilla Valerius.

He is an Archery skill trainer, and a potential companion if you side with him by exposing Sven's letter to Camilla.

Exploit
An opportunity to gain a large amount of archery skill and level quickly very early on exists with this companion. Complete the side quest by exposing Sven's letter, and then speak to Faendal. You will be able to recruit him now as a companion -- ask him to follow you. Once this is done, pay for archery training.

Then, use the function to trade items with your companion. You will quickly realize that your gold that you just paid for training with can be taken back from his inventory without complaint. This process can be repeated up to the training-per-level cap of 5, the only limitation is a price increase in 10 gold per training session. You'll still get your money back after each repetition, this just means you have to have a little extra money to pay for the not-quite-free training. You should begin this process as early as possible.

It stands to reason that other companions who are trainers can also be exploited in this way, at least until the likely patch closes that door (by, for instance, preventing gold used to pay for training from showing up in the companion's inventory afterward). However, Faendal is a confirmed case of this exploit.