Offset Missionary: Enter her in the missionary position as usual, lay flat against her, have her lay one leg flat & slide it inward so it's between your legs, screw ;). This lets the guy get in especially deep & feels very good for both partners - a harder part of the guy's pelvis is grinding against her clit than in the regular missionary position & the guy's balls rub against her soft inner thigh which feels really, really good!
G-Spot Killer: Well, this gets my wife's g-spot anyway...she lays on her right side & lifts her left leg up. I straddle her right leg while on my knees & enter her. I support her raised left leg by either holding her ankle or letting her heel rest on my left shoulder. My wife loves this, of course, because it hits her g-spot. I love it because my hands are free to fondle her tits & ass while I fuck her and, again, my balls are rubbing against her inner thigh.
Extra Deep: The 'lunge' part of this always makes my wife gasp...she lays flat on her belly on the bed with legs spread a medium distance apart. Get on her supporting yourself straight-armed with a hand on either side of her neck (to keep her from slipping away while you ride her). Stick it in while resting your knees above her thighs & your shins pinning down & spreading the back of her knees. Once she's good & warmed up, slowly lunge you whole body forward, towards her head while your cock is buried as deep as it can be & keeping her legs pinned down - this should give you both the sensation of extra deep penetration.
Again, please let me know if you like any of these - I'd love the feedback!