Well, it really depends on what type of Prius you have.
If you have a NHW11 "Classic" 2001-2003 Prius, you are really limited for tires because of the weight designation.
According to US TSB PG015-00 dated Oct. 13, 2000, the only tires that Toyota USA recommends for the 2001 Prius is the Bridgestone Potenza RE92 XL, and for winter/snow tire use the corresponding size/XL Bridgestone Blizzak tire.
Unfortunately, the Potenzas have a very low treadwear rating of 160, so they don't have a long lifespan (usually only 20,000-30,000 miles, though I have heard of some at 8,000 out to about 45,000 before too worn to use). Rotate at every oil change, and maintain the factory 35psi front/33psi rear as a minimum tire pressure... (For longer tire life, many suggest pressures higher than this, up to the max. cold pressure listed on the tire's sidewall, with 2 less psi in the rear. For the Potenzas, the max. cold pressure is 50psi, and many owners (including myself) like it in the 40psi range (42f/40r, 40f/38r, etc.). The reason for the higher pressures is that the Potenzas/Prius are prone to rapid edge wear, indicating too low of a pressure...)
anyhow, if you are looking for replacements, it would be best to keep the same size and especially the load rating if at all possible. Some tire dealers won't sell you a normal P175/65R14 tire, which usually only has an 81 or 82 load rating...
US 2001-2003 NHW11 OE Prius tire:
Bridgestone Potenza RE92 XL (Extra Load)
P175/65 R14
84S
Treadwear 160
Temperature A
Traction A
50psi max. cold pressure
Canada 2001-2003 NHW11 alternate OE tire
Dunlop SP10 XL
P175/65R14
84S
treadwear 220
traction A
temperature B
50psi max. cold pressure
(treadwear rating on the Dunlop SP10 XL isn't much better than that on the Bridgestone Potenza RE92 XL, so not much better on longevity, but a decent replacement now that the Potenzas tend to be on backorder...)
a good alternate is:
Nokian WR
P175/65R14XL
86T
Severe Service Emblem (the only all-season tire on the market that is also a snow tire (denoted by this emblem), to my knowledge)
(I think:
400 treadwear
A traction
A temperature
I don't recall if it's 44 or 51 psi max. cold pressure)
Do check the Prius lists for more info, as I think that you may still be able to get in on a Nokian group buy discount from Ryan through tirefactory.net. He may also suggest another tire in the Nokian line if you don't see much ice...
and a tire that I don't have a lot of info on, other than to know that it is in the Continental brand's family line:
Barum Brillantis
175/65 R14 XL
86T
more tire info at: http://john1701a.com/prius/prius-tires_classic.htm
Other alternatives (usually in different sizes to allow for the load rating) that seem to be mentioned more often as possible alternates (other than getting a new set of the OEM Bridgestone Potenza RE92s) that ring a bell as I look over the tire database below (that aren't winter tires to my recollection):
Dunlop: SP10, SP20, Sport A2
Goodyear: Aquatred3, Integrity, Allegra
Michelin: X-One (and some Costco? version), Symmetry, MX4, Hydroedge
Nokian: NRT2, i3
Pirelli: P[some #]
Toyo: 800 Ultra
I'm certain that if you try searching the Prius owners' group archives on some of the manufacturer's names, you'll come across someone who's tried them...
If you need to go up to P185/65R14 size tires to find the load rating in the brand/model of tire that you like, it will fit on your Prius, although your dealer may not like it.
You may also want to check out the tire database in the main Toyota Prius group. More owners are at: http://autos.groups.yahoo.com/group/toyota-prius/ (the dash makes a difference!)
Database: There is the Alternate tire Database: http://groups.yahoo.com/group/toyota-prius/database?method=reportRows&tbl=14
Other places to check: http://www.priuschat.com/ and http://www.priusonline.com/
(The one unfortunate thing about tirerack.com 's reviews and tire info is that it only covers models that they stock...)
If you have a NHW20 2004-current Prius, you have a lot more options as you don't have the same weight restriction. However, the size depends on what type of Prius you got and the option package offered...
US/Canada 2004-2008 NHW20 OE Prius tire:
Goodyear Integrity (Standard Load)
P185/65 R15
86S
Treadwear 460
Traction A
Temperature B
44psi max. cold pressure
US 2004-2006 NHW20 port-added 17" Prius tire:
Pirelli "P Zero Nero"
P215/45ZR17
87W M+S
treadwear 400
temperature A
traction A
51psi max. cold pressure
US 2007-2008 OE NHW20 Touring edition tire:
Bridgestone Turanza EL400
P195/55R16
86V
treadwear 260
temperature A
traction A
51psi max. cold pressure
Again, if you want a true 4-season tire, the Nokian WR A.W.P or the newer Nokian WR G2 comes highly suggested.
If your currently worn Goodyear Integrity tires haven't caused you any problems (once worn a little, they tend to ha