• The Basics
• Eyedroppers
• Retracting-nib safeties
• Lever-fillers
• Button-fillers
• Piston-fillers
• Crescent-fillers
• Twist-fillers
• Bulb-fillers
• Pump-fillers
• Vacumatics
• Plunger-fillers
• Blow-fillers
• Touchdowns/Pneumatics
• Snorkels & PFMs
• Capillary fillers

The simplest self-filling pens conceivable, capillary fillers have no moving parts and are filled by being dipped in ink.  The most common capillary filler is the Parker 61, shown above.  After unscrewing the barrel, the teflon-coated reservoir inside is dipped in ink. 

The less common Waterman X-Pen, on the other hand, fills by immersing the nib end (some pre-production Parker 61 prototypes used this method as well).  Capillary fillers are vulnerable to clogging of their reservoir units, so don't let them dry out and avoid especially heavily-pigmented ink.  In most cases, however, thorough soaking and flushing under pressure will restore the pen to working order.