In no particular order:
1. High Level Alchemy (55 magic)
The horrors! High Level Alchemy is one of the most well-known portable methods of training. It's easy to do, it can be used on almost anything, and it is a primary method of training magic. Although some minigames prohibit the usage of this spell (Trawler, Pest Control, Castle Wars, etc.), it can be used in some others. Don't sit there in a bank alching, play a minigame while doing so!
2. Bolt Enchantment (4+ magic)
Quite similar to high alchemy, this method is also portable and you enchant bolts instead of converting items to coins. The difficulty is slightly higher as the required materials are not easy to come by. However, it is faster magic experience than alchemy and is allowed in the minigames where high alchemy is prohibited. It can actually even profit in rare cases, unlike High Alchemy.
3. Arrow fletching (1+ fletching)
Fletching arrows either by adding feather to shafts or adding tips to headless arrows grants fletching experience, and is one of the primary portable non-magic training methods. Higher levels let you use better tips and therefore yield better experience. It is allowed in most minigames.
3.1415926. Bolt fletching (9+ fletching)
Essentially the same, but you add feathers to unfinished bolts instead. Mithril and broad bolts are popular for this purpose.
4. Fishbowl + Humidify (68 magic, Lunar Magicks required)
This method requires 2 fishbowl and humidify runes. First, empty one fishbowl (it's left click as opposed to right click on buckets, so fishbowls are better), humidify, empty the other, humidify, repeat. It is extremely fast magic experience, but with 100% loss. Magic imbue may supplement the magic gain as it may be cast once per 20 seconds. [Thanks Blizaga57
5. ???
Please post other methods you know of here for updating




