The first is the Data Transfer utility that gets put into Excel when you install iSeries Access. See:
"How do I add the Data Transfer from AS400 option to Excel?" at
http://faq.midrange.com/data/cache/194.html

Another is to use CPYTOIMPF and/or CPYTOSTMF, or for the other way, CPYFRMIMPF and/or CPYFRMSTMF. These will transfer data from DB2 into the IFS. Quite easy to make a CSV file to port between Excel. There are issues that some people have with it, especially after V5R3, so many have started reading and writing to the IFS directly. A couple of references on doing the programming for this may be found at
"Who Knew You Could Do That with RPG IV? A Sorcerer's Guide to System Access and More" at:
http://www.redbooks.ibm.com/abstracts/sg245402.html

Another good resource is "Reading & Writing from the Integrated File System" by Scott Klement available at: http://www.scottklement.com/rpg/index.html

An amazing resource for the highlest level of granularity in creating and/or updating an Excel file, including column formatting, multiple worksheets, and all your fantasies come true is "RPG and Excel with Java" at:
http://faq.midrange.com/data/cache/533.html

And, even though we've decided to create this faq, this question will probably still pop up on the lists about weekly. Therefore searching the archives may not be a bad idea.
http://archive.midrange.com/

There are some freebies to assist:
CRTCSVF at
http://homepage1.nifty.com/uzaemon/index.html
Don't be frightened at the foreign language at first, scrolling down reveals english.

http://www.geocities.com/~alex_nubla/tips.htm

http://archive.midrange.com/midrange-l/200609/msg00021.html
http://www.mcpressonline.com/mc/1@175.x1YUb27p2Cz.5@.6aed3f76

There are several commercial resources available:

One of the RJS Software products (I think RPG2SQL) at
http://rjssoftware.com/

Kemetech Conversion Suite at:
http://www.kemetech.com/