(1) Marriott to AA, 4 times, 120,000 mile increments. I wouldn't be Lifetime Gold on AA without Marriott.

(2) Marriott to WN, 80 credits. We had an upcoming year where I knew we'd be flying to SFO, SAN, SEA, BWI, CHI (a few times), and DEN (a couple times). Paired up the Marriott credits with some Alamo car rentals to hit 100 credits - flew nearly all of my domestic U.S. travel that year for free, paying only for the cheap MDW and DEN segments where it didn't make sense to burn awards.

(3) SPG to YX, 5000 miles. (I'm trying to recall...I might have used Amtrak as an intermediary on this one.) I needed a one-way ticket to Duluth and this was by far the cheapest way to get it - a YX award seat. (In general, I am loathe to convert SPG points - I find them more valuable used for hotel stays.)

(4) AA to HH, I forget exact amount but it was around 75,000 AA miles. Actually followed close on the heels of one of the MR-AA conversions, prior to a one-month trip where I redeemed two Marriott Travel Packages and 2 HH 6-night awards.

(5) Going oldschool to the 1990's: Numerous Diner's Club conversions to miles and points. I recall doing AA and BA conversions doing promotional periods.