Megabus is a budget coach division of Stagecoach. It offers cheap coach fares on a range of routes - for example around £6 from Newcastle to London.
As the information provided about stops is a bit vague (i.e. street the stop is on but often not where on that street) here are some more specific details about location etc. (please add details of any others you have used):
Where: London Victoria, Greenline Coach Station. Over the street from the main Victoria coach station.
Info: The bus departs from London Victoria Coach Station, usually from inside the garage. Workers are always around, should you need any assistance.
Where: Neville Street (street Central Station is on), stop near Royal Station Hotel end (as opposed to stops near car park)
Info: No shelter or seats
Where: St. Catherine's Park and Ride, near Bar End Rd.
Info: Buses stop near public toilets building. Benches and seats in bus shelters. Buses available to centre of Winchester.
Where: High Street, down towards Town Quays end, near small green patch, further down from 'Old Mayors Cellars' (or similar)
Also stops on Burgess Road (near to University of Southamptom Highfield Campus).
Info: Shelter with seats available or can sit on adjacent grass (High Street)
Where: Main bus station
Info: Shelter with seats available, also cafe, toilets, etc. available inside the bus station building.
Where: Perth Park and Ride on the western side of Perth, near the M90, A9 and the broxden roundabout. Or Perth Bus Station located in the city centre, near train station.
Info: Park and Ride - One bus shelter available, though because around 4 megabuses pass through here at about the same time, it's usually difficult to get a seat. Bus Station - Toilets, newsagents on site.
Where: Buchanan Street Bus Station - usually stance 52. Info: Plenty of indoor seating. Don't be put off if it looks like the bus is going to a totally wrong city - many journeys involve changing at Perth. There are pay toilets at the bus station, but they are currently closed and have been replaced with portaloos.
Where: St Andrews Square bus station. Close to the Waverly end of Princes Street.
Info: Major indoor bus station, so plenty of seating and loos and such. There is another drop off point in Edinburgh on Queensferry Road.
Where: Outside Chorlton Street Coach Station
Info: The bus stops at the "free bus" stop outside the station, not in the National Express station itself. If you have time before your bus arrives, there are paid (20p) toilets, hot & cold drink and food vending machines, and a shop which is half "Upper Crust" and half paper shop. Should be enough to pass the time.
Where: Kingsway y Fford Brenin.
Info: It does not stop at Cardiff Central Bus Station. The stop is on a small road between Boulevard de Nantes and Greyfriars (opposite the Hilton Hotel). Journey time to London is about 3 hours 15 minutes and costs about £5-7.
Where: Aberdeen bus station
Info: The bus station is attached to the Union Square shopping centre, so there are plenty coffee shops, toilet facilities and seating available. Union Square is in turn attached to Aberdeen railway station and is a short walk from the NorthLink ferry terminal.