I've recently been playing around a lot with saved game files after reading the Gran Turismo 5 thread, so thanks to RiPPERD for the information in their thread.
This is fairly easy to do (and once you've done it a few times, it gets even easier).
What you need (blue is what you need on your PS3 and red is for your PC:
CFW (any will do, as long as you can install packages)
Blackb0x FTP 1.2
WinSCP (or equivalent)
SFO Editor (I use the version 0.80)
HxD (or any other hex editing program)
I cannot guarantee this will work for every game as I do not have an infinite amount of time to test, but I have 20+ PS3 games atm that I can certainly do some tests on. This thread may be updated with future information
Find a game you want to use a save for (I will be mentioning FF XIII throughout this thread as a reference game, I've also tested this on Gran Turismo 5) and you must also be able to find other people's saved game data for it. I use GameFAQs.
Also keep in mind that if saves are copy-protected you likely won't find them online easily as someone would need CFW on their PS3 to snatch those save files and post them.
Copy your own save for said game to your PC via FTP so you can grab your PSN ID from the Param.sfo (will be explained).
(a)Open PS3SFOEdit and ignore the prompts saying it's in Beta and all that (we don't need to use this program except to grab one line of HEX code).
(b)Click "Load" and browse to the save game you copied to your PC and open the PARAM.SFO file of the save.
(c)Click "Advanced Edit" and look at Line 1 (ACCOUNT_ID). Click the number, wait 1 second and then click it again (which will make the HEX selectable). Press CTRL+C to copy the HEX value.
(d)Open Notepad or any Text editor and paste (CTRL+V) your ACCOUNT_ID into it so you don't forget it.
(e)Go back into PS3SFOEdit and close the Advanced Edit screen. Click on Load and browse to the PARAM.SFO file of the save that you have downloaded from someone else (or from GameFAQs).
(f)Click "Advanced Edit" again and look at Line 1 (ACCOUNT_ID). It will be different than yours, do the same as step (c) and paste it into your Text editor noting that this is the other person's ACCOUNT_ID and not yours.
(g)Close out of PS3SFOEdit and open HxD (or any other HEX Editor) and load the downloaded save's PARAM.SFO.
(h)Press CTRL+F to do a search, and paste their ACCOUNT_ID into the text box, making sure to select Hex-values for the Datatype. Click OK and it will find their ACCOUNT_ID and highlight.
(i)Go back to your Text editor and select your ACCOUNT_ID and press CTRL+C to copy it, then go back into HxD and with their ACCOUNT_ID selected press CTRL+V to paste your ACCOUNT_ID into it.
(j)Repeat step (h) as the ACCOUNT_ID is in the PARAM.SFO file at least two times, and then do the same as step (i), putting your ACCOUNT_ID over theirs.
(k)Repeat step (h) as a precaution (I haven't found any PARAM.SFO files in the Savedata that have had the ACCOUNT_ID listed more than twice, but better to be safe than sorry).
(l)Make sure you click File-> Save to save the changes you made (or CTRL+S).
Run Blackb0x FTP on your PS3 and on your PC load up WinSCP and FTP into your PS3. Browse to hdd0/home/00000001/savedata (note that 00000001 may be different for you depending on how many users were created on your PS3 and which one you use, but I'm sure you guys can figure that out).
Browse to the folder MRTC00003-02 (for Final Fantasy XIII, the '-02' part is in reference to what save slot it is in game, it can be any save you want though, add +1 to the number and that's what save it is in game, as the savedata folder starts them out at '-00' for this game).
Delete the contents of the folder and copy the downloaded save contents to that same folder.
Close out Blackb0x FTP and load up your game, load the Save, and voila!.
If I have missed any steps, please let me know.
I have found this to be a much easier way to try someone elses save out without worrying about whether it will work or not as transferring via USB will usually give you an Error (even though forcing the save to the PS3 via FTP will work just fine).
I have tested this with the save data in the Gran Turismo 5 thread (I even went to GameFAQs and downloaded someone elses save there and tested it, and it worked).
I downloaded 3 different FFXIII saves and they all worked for me.
Please let me know if this was any use to you, and if you've tested it on other games I would like to know how they worked.
If you have any questions, feel free to ask, and I will try my best to answer them.
I have uploaded a RAR file for anyone that doesn't have SFOEditor and Blackb0x FTP 1.2.
Multiupload.com - upload your files to multiple file hosting sites! - Password is "Satanicat" without the quotes.
I can include pictures for this tutorial if enough people want me to, but it shouldn't be necessary.