As mentioned before, Ricko is a good source of scale cars, including some nicely detailed classics that fit right in with most steam eras. If Rolls Royce is your thing, for the country club set, Praline has a nice set. And since I 'live' on eBay, a quick plug for a store called 'WeHonest' (yeah, yeah, I know - but they ARE..), that sells slightly smaller cars (1:100), outfitted with LEDs front & back for around a buck each. I got my motion-controlled track signals from them, too.
Matchbox/Lesney Rolls-Royce Phantom V is a huge 1:43...
As mentioned in many replies Hot Wheels are mostly 1/64the scale, the "HO" Slot cars are about 1/64th scale and Matchbox are all variety of scales.
As for Matchbox this quote "This famous, originally English, manufacturer of die-cast models has always released models that collectors often refer to as 'box-scale', i.e. the models had to fit into the boxes they were sold in and the actual scale of the model was of secondary importance. There were, however, several models that are actually very close to 1/87." Came from this page http://www.87thscale.info/matchbox_ii.htm
Hot Wheels has never officially declared their cars to be any specific size, except on occassion they will run a limited series of labeled to scale cars and they did this briefly in the Mid 2000's in 1/87 scale, presumably produced in the same China factory as the Malibu Classics/Fresh Cherries/Model Power cars and just branded for Hot Wheels. Like the Malibu and Fresh Cherries they were available at Wal-Mart and Toys R Us for under $2 each. Hot Wheels 1/87, Fresh Cherries and Malibu are no longer being produced in 1/87th scale although the Malibu line is was icensed/bought by Model Power and can be found for anywhere from $5 to $10 per car.UPDATE since originally writing this I have found out Model Power suspended business and had now struck a deal with MRC to produce and market some of their products. There are also two other under $10 lines "Welly" and "High Speed" but I have not been able to get confirmation either of these are still in production although I heard Welly has a deal with Atlas so they may still be around.
The slot cars were labeled as HO Scale in a marketing ploy for people to buy them with their train sets, and in fact Tyco, Life Like and others put out HO train and slot cars sets with crossing tracks. The cars are much closer to 1/64th scale and originally all of them were made to fit the same size chassis so that is why an Indy car looks as big as a NACSCAR.
EBAY ALERT. There is another company named Maisto that has many retail and internet (especially ebay) models for sale and NONE, let me repeat NONE are 1/87 scale....be particularly careful on ebay as these are often misrepresented (as are the Matchbox and Hot Wheels) as 1/87 in the Ebay ad but again they are NOT 1/87th scale. I have emailed Maisto and they assure me they have NEVER put out any models close to 1/87 scale!
Hope this clears things up a bit.
Just what i was looking for, i just bought some Maisto from dollar tree and was wondering if they would working with HO scale buildings and figures, i have hot wheels but i dont want to open them.