The Stirling Uni shuttle run by First is "numbered" UL
Outwith the UK:
In Innsbruck, city bus services are lettered*, whilst tram services are numbered (two notable exceptions were bus service 4 (Now 504 - Innsbruck to Hall in Tirol, former tram service 4) and the Stubaitalbahn, which is 'STB' (City running and overland Tram up the Stubai Valley to Fulpmes, formally a seperate system from the city trams and ran without any route number))
* Bus services that were lettered, which run outwith Innsbruck are now numbered
D - 501
E - 502
S - 503
(4- 504)
ST - 590 (Stubai Valley bus)
Vienna still has 2 of it's lettered Tram routes remaining the D and O
Vienna also used to have tram routes 5/31 and 31/5 (placement journeys which were part of routes 5 and route 31), likewise, 60/62 was another
I remember when I was in Rome many moons ago, all the suburban bus routes that didn't go into the city centre were all 0xx route numbers
Similarly, many American cities opt for route letters instead of numbers. Examples include Tallahassee, Davis (California), and State College (Pennsylvania).
Two major cities have both numbered bus routes and lettered bus routes - Philadelphia and Miami. In the case of the former, this is because the PTC (the city's former transit operator) used letters for bus routes and numbers for streetcar routes and some bus routes managed to somehow avoid having their letter replaced with a number after SEPTA took over. In the case of the latter, it is because the previous operator in Miami Beach used letters for its bus routes and Miami-Dade Transit kept the letters after it took over (though they also use numbers in the 1xx series on these routes alongside the letters).
WMATA (Washington) has possibly the most confusing numbering scheme of any American agency. The rough breakdown is as follows:
Numbered routes - Old streetcar lines
Alphanumeric routes starting with a letter (example - N2) - Other bus routes in DC and Maryland
Alphanumeric routes starting with a number (example - 38B) - Bus routes in Virginia
Sometimes, all routes starting with one digit or letter share an alignment (examples include the N-series buses along Massachusetts Avenue and the 7x series buses along Georgia Avenue). In other instances, routes with similar alphanumeric codes are nowhere near each other (the best example of this is the B-series buses, which are mostly concentrated in the suburb of Bowie, but there is also a B-series route within the district proper).
The most interesting scheme for naming routes, however, has to be Rapid City, South Dakota. Here, bus routes do not have numbers or letters - they're named after famous people connected to Mount Rushmore.
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