There are many MLP pipeline companies. Your best bet is to check out the company website to see which process they are manily involved - some are gathering/processing, some are "long haul" but they may also own processing stations.
Some examples are: Boardwalk Pipeline Partners (BWP); Buckeye partners (BPL), Enbridge Energy (EEP); Energy Transfer Partners (ETP); Enterprise Products (EPD), Kinder Morgan (KMP/KMR); Magellan Midstream (MMP), Nustar Energy (NS); Oneok (OKS); Plains All American (PAA); Spectra Energy (SEP); Williams Partners (WPZ).
Gatherers/Processors include: Atlas Pipeline (APL); Copano Energy (CPNO); Crosstex Energy (XTEX); DCP Midstream ((DPM); Eagle Roak Energy (EROC); Markwest Energy (MWE); Regency Energy (RGNC); Targa Resources (NGLS) and Western Gas (WES).
Then there are companies that do exploration and production, but also own pipelines: Linn Energy (LINE) has a small network of pipelines; PVR Partners (PVR).