TSO, or not TSO, What are the Requirements?
On the mailing lists there are lots of discussions about TSO or "certification" requirements for instruments and avionics in an experimental aircraft. Basically, the answer is, NOTHING in the panel needs to have an actual TSO in order to be legal in an experimental aircraft.
Here's why:

For reference, FAR 91.411 can be found here:
http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/58128BDB26BDF59486256DC000521732?OpenDocument
and 91.413 here:
http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/35329CD67B08E3FD86256DC0005217C8?OpenDocument
and 91.205 here:
http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/F03940D26321E1DF86256F7F0061FFAB?OpenDocument
Part 43, Appendix E:
http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/6D8DD8E42FAAB642852566AB006BCD8B?OpenDocument
and F:
http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/9AC8CCE87ACBDA80852566AB006BCECB?OpenDocument
and 91.215:
http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/5EE8F0240FD4DD3986256A6900532C8A?OpenDocument
and 91.217:
http://rgl.faa.gov/REGULATORY_AND_GUIDANCE_LIBRARY%5CRGFAR.NSF/0/83B7AB36D06BAD60852566CF0067A8D1?OpenDocument

Note that although 91.205 starts out with "Powered civil aircraft with standard category U.S. airworthiness certificates" which may lead one to believe it does not apply to aircraft with a special airworthiness certificate that our experimental aircraft have, in the operating limitations attached to the special airworthiness certificate, there is likely wording such as

"After completion of Phase I flight testing, unless appropriately equipped for night and/or instrument flight in accordance with 91.205, this aircraft is to be operated under VFR, day only".

thus making it apply to us if you are going to be flying at night or IFR.

You will generally also find wording in your operating limitations such as:

"Aircraft instruments and equipment installed and used under 91.205 must be inspected and maintained in accordance with the requirements of Part 91", which makes 91.411 and 91.413 apply to us as well.

91.413 refers to transponders in reference to 91.215(a), which says that "ATC transponder equipment installed must meet the performance and environmental requirements of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with altitude reporting capability) as appropriate, or the appropriate class of TSO-C112 (Mode S)."

Note that technically the transponder does not have to be TSO'ed, but it has to meet the requirements of the TSO. For all intents and purposes, easiest for us just to use a TSO'ed transponder than go through the TSO testing on our own... :-)

So, other than the transponder having to meet the TSO requirements (but not actually having to be "TSO'ed"), no other instruments or avionics (such as the altimeter) would seem to have to be TSO'd according to these FARs. There are lots of IFR-legal experimentals flying with non-TSO'd glass panel EFIS systems such as the GRT or the Dynon. As long as those EFIS systems can pass the required tests, they are apparently legal to use.

An excellent in-depth article on this topic can be found on page 6 of the November 2003 Van's Air Force Homewing newsletter that you can read at this link.

There is a compelling discussion on the GPS requirements for IFR here: http://www.matronics.com/searching/getmsg_script.cgi?INDEX=45661616?KEYS=gps_ifr?LISTNAME=AeroElectric?HITNUMBER=1?SERIAL=19092528726?SHOWBUTTONS=YES