I like to use SkyVector for flight planning - it has both VFR and IFR charts of varying quality around the world. Note that it's primarily intended for real-world pilots, so if you choose to make an account, be careful that you don't file a real flight plan 🙂
If you're flying in the United States, you can download all the official VFR charts ('sectionals') straight from the FAA's website. Each download covers the area of a few states, and is usually about 200MB in size. VFR charts for other countries are a lot harder to come by - if you want to see what I mean, go to the US-Canadian border in SkyVector's VFR mode, and observe how suddenly the level of detail drops! VFR charts for the UK are only available from specific publishers as far as I can tell, and they are quite pricey.
Try ChartFox for taxiway diagrams for specific airports - it has ICAO taxiway diagrams for what appears to be every British airport, and old Jeppesen charts for Canadian airports which can be a bit 'hit or miss'.