Chargers I recommended

• Delkin Devices DD/AA-QKCHGR (2007).  A travel charger with four independent channels, but at $45 this is way more than the one I recommend.
• Duracell:
• Value Charger CEF14DX4 (2008).  Not smart, shuts off after a certain amount of time.  See "Smart vs. Dumb Chargers" for why this is bad.
• Mini CEF-20 (2007).  Smart charger, 100-240V AC, but charges only 2 cells at a time...slowly (280mA each for AA, 110mA each for AAA).
• Mobile CEF23 (2008).  A good charger, but for a travel charger there are much cheaper, and for a workhouse charger there are much better, and this model was discontinued anyway.  A nice feature was that it powers USB devices, in three different ways (from the AC adapter, inserted batteries, or the car adapter).  It's smart, charges 1-4 batteries independently (each with its own status LED), and takes 100-240V input.
• Go Mobile CEF26 (2009).  Universal AC, charges in pairs only.
• Energizer Value Charger (2010).  Not smart, and requires charging in pairs.  The older version was manual, and the newer one is timer-based.
• GP Recyko Value Charger (2010).  Dumb charger (charges forever until you remove the batteries).  Must charge in pairs.  
• Kodak K6200 (2001). Smart, 4-cell, 4-channel travel charger, with 100-240V input.  But it's been discontinued, and its replacement the K6600, charges only in pairs, natch.
• Panasonic BQ-390 (?).  Smart cutoff, independent channels, but only 1 status LED.  100-240V input.  No longer available.
• Rayovac Easy Charger (2010).  Confusingly, there are two different chargers sold with this name.  One is dumb, and will try to charge the batteries forever (damaging them).  Both models charge only in pairs.
• Sanyo Eneloop Chargers:
• MDR02 (2003).  Smart charger, charges each cell separately, but accepts only 2 batteries at a time (not 4).  100-240V input voltage.
• NC-MQN04U.  Charges only in pairs.
• NC-MQN05U.  Charges 1-4 batteries individually (smart).  Discontinued.
• NC-MQN06U & MQN06W (2009).  Charges only in pairs, and charges slowly.  The -06U has a fold-down plug and the -06W uses an AC cable.
• See all the Sanyo Eneloop chargers 
• Sony Cycle Energy BCG34HLD4KN (2003).  Smart, charges each cell separately, 100-240V, but pricey at $24.
• Targus Digital TG-LCD2700 (2003).  A good, smart, 4-cell, 4-channel travel charger, with 100-240V input and 12V input via a car adapter.  But it's been discontinued.

Other 8+ SLOT CHARGERS (charges up to 8 batteries unless otherwise noted)

• Lenmar Pro78 (2004).  Timer-based, with a 2000mAh limit.  Requires charging in pairs.
• Maha Powerex MH-C800S (2001).  Smart, eight independent slots, worldwide power supply, LCD shows status of each battery, can choose between soft or fast charge mode, refresh option.
• Tenergy TN157 (2011).  Smart cutoff, eight independent slots, refresh mode.
• Tenergy T-6988 (2008).  Smart, ten independent channels.  Input is 110VAC or 12VDC.  Costs more and charges fewer cells than the one I recommend.
• Titanium MD-1600L (2011, $50).  Charges sixteen (!) batteries at once...independently.  Smart cutoff.  Also charges USB devices.  Can operate with car adapter. 
  The 12-battery model in my recommendation table is a little cheaper, but if you need to charge 16 batteries at a time, this is the charger for you.
•  SUNJACK USB BATTERY CHARGER