What is a good power levelling strategy for Diablo 2?

Equipment is largely irrelevant in terms of rushing - while you're rushing, if you can't handle a monster, just back off and let others kill it. In general, unless you have someone specifically rushing for you (as mentioned in these other answers), you'll only have to worry about waypoints and the staff in Act II. You might need help with the Ancients and bosses, but you can usually find a game for that. So if you can get someone to give you the waypoints you need (and the staff...), all you really need to worry about is where you're receiving the best experience. For the best experience with very little resources, I follow this procedure:

• Levels 1-15: Normal Tristram runs
• Levels 16-20: Normal Tomb of Tal Rasha runs
• Levels 21-24: Normal Cow Level or Chaos Sanctuary runs
• Level 25: Normal Ancients to gain 1 level, achieving level 25
• Levels 26-50: Normal Baal runs (somewhere in the 40s people usually transition to Nightmare - you can't do Nightmare Ancients until 40, but you get decent XP from Normal Baal runs until about 50)
• Levels 51-60: Either get through Nightmare and do Ancients then Baal runs until 60, or get through Nightmare and Hell and do Hell Chaos runs
• Levels 60+: Hell Ancients, then Hell Baal runs

Alternatively, some people get bug-rushed all the way to Hell and then do Hell Chaos until level 60.

As far as rushing goes, you can often get someone to rush you through a difficulty by offering them the drops from your Forge (Act IV, Quest II - destroying Mephisto's Soulstone).

There used to be a concept called "ubering" (which is the most expensive, but also the easiest). It involves getting a few high-level characters together to kill all the Ubers in Uber Tristram except Mephisto, then get him stuck in a house while minions continue to spawn. If you get a few Hammerdins and their mercs together, you can basically sit AFK while they kill everything (by holding down Shift + Left Click, using a script/bot, etc.). This allowed for 5-6 low level characters to gain significant numbers of levels quickly. The explicit process is available here.

Ubering (also called uber leveling or UT leveling) is no longer available as of 1.13 (it appears Mephisto's minions no longer grant experience).

While leveling in Diablo II it is important to remember that until you get a character to level 25 it is best to try and only kill monsters that are within three levels of yours. For example, if your character is at level 10, try and stick to killing monsters that are in the 7 to 13 range(reference). This is due to the severity of penalties that you get to the amount of experience netted before level 25.

After level 25 it is best to always kill as high a level of monster as you can safely do so.

Additionally, it will certainly not hurt to have a good build guide to help you in the process of building a character that is strong enough to make it to level 90 and beyond.

It would seem that this guide is succinct, but requires multiple players:

Using the Classic Rush method:

1. Do a classic Rush through all difficulties

2. Once you are in hell Act 3,
   convert your character into an expension char.

3. Kill Mephisto and start doing Chaos runs till level 60.
   Good experience starts at level 25.

4. On level 60 start doing Baalruns.

5. Do Baalruns till level 95.

6. Start doing Diablo and Baal.

Alternatively if you do not have a classic friend.

1. Rush to Act 5 normal.

2. Find a game where someone needs to do the Baalquest.
   Wait in town (you can not enter TP cause you have no ancients).

3. Let him finish the baal quest.
   As a party member you complete the quest too.

4. Repeat this for nightmare.

5. In hell do Chaos runs to 60.

6. Baal runs to level 95.

7. Starting there, again Diablo + Baalruns till 99.

To be honest, this is an enormous question to ask - there are so many factors that are involved that affect the speed of your leveling. I unfortunately do not have the time it would take to detail everything as precisely as I would want to, but here are some general things to keep in mind:

1. The more players you play with, the stronger the monsters get and the more EXP they give when killed. This is why you see people constantly killing end bosses for EXP in a 8-player game.

2. It is important to kill monsters that are near the same level as you, with a party that is also similarly leveled. The EXP distribution across the party isn't that great when there are large differences in either the monsters, or the party members. This is the normal case for leveling (especially when playing by yourself), but see point #4.

3. Higher difficulties/areas do indeed yield more EXP as the monster's levels are higher.

4. After rushing (either classic rushing, normal rushing, or glitch rushing), the idea is to level up to a comfortable 75-80 range, and then go back to lower difficulties to find equipment to wear/use. You will undoubtedly find practical pieces of equipment while this leveling occurs as well.

So for rushing, you may be wondering how exactly you'd be able to level like this so fast and with no equipment. Basically, you're reliant on public games that have a human or a bot running through a Hell-difficulty end boss to do the killing for you, while you sit there and leech EXP.

These days, the most popular way to level is to glitch rush from level 1 to Hell Act 4, sit in Hell Chaos Sanctuary run games until level 60 (minimum level to do Hell Ancients), and from there do Baal runs until you're satisfied.

Please see FCTW's answer if you're looking for specifics on how to either Classic Rush or Glitch Rush. Normal "rushing" is straightforward, and aspects of normal rushing are done in both Classic and Glitch rushing.