In ICE's HARP System: After level-up I bought skills, stats etc and had one DP left over. Can I carry it over to the next level up? Alternatively, is there something that can be gained with one point?
DPs Must Be Spent When Earned
On page 17 of the rulebook:
All Development Points must be spent when they are gained; any unspent Development Points are lost. Many talents and options may be purchased across multiple character levels, meaning a character may begin to pay for a Talent at one level, and then meet the remaining cost on the subsequent level or levels later on in their adventuring career.
So, you must spend your 1 leftover point at the time you gained your new level, however you can allot that point towards the next skill or talent you'll wish to buy at your next level up, which will make that purchase cost 1 DP less at that time.