Here are well-known/top software engineering firms and tech delivery centers in Sri Lanka (strong engineering centers, product companies, or large delivery hubs). I list each with a short note and a source you can check for details.

• WSO2 — Global open-source API/integration and identity platform vendor headquartered in Colombo (large engineering center serving global customers). (WSO2.com)
• Virtusa — Global digital engineering & IT services firm founded in Sri Lanka with major delivery centers (large Colombo engineering presence). (en.wikipedia.org)
• LSEG Technology / MillenniumIT — High‑performance trading and capital‑markets technology (founded in Sri Lanka; part of LSEG / large Sri Lankan engineering operations). (elfaonline.org)
• Sysco LABS (Sri Lanka) — Captive engineering/innovation centre for Sysco (Fortune 500) that builds large-scale product and platform engineering in Colombo. (nz.linkedin.com)
• 99X Technology — Product engineering specialist from Sri Lanka (strong track record delivering software products for international ISVs). (99xtechnology.com)
• Calcey Technologies — Agile product engineering and software services with a major delivery centre in Colombo (serves startups and enterprises internationally). (ten.lk)
• MillenniumIT ESP / MIT ESP — Local systems‑integration & enterprise solutions firm (enterprise platforms, managed services; distinct from LSEG Technology but also a major local tech player). (en.wikipedia.org)
• John Keells IT (JKIT) — IT arm of John Keells Holdings delivering software/solutions (notably in travel/airline and enterprise systems). (en.wikipedia.org)
• hSenid Software — HR/payroll and enterprise software product company with development in Sri Lanka (well‑known local product vendor). (onedigitalsl.com)
• CodeGen / Zone24x7 and other notable local firms — Sri Lanka also hosts several strong product‑engineering and services companies (e.g., CodeGen, Zone24x7, Zone24x7/others) and many smaller specialist consultancies and startups. (onedigitalsl.com)

Notes:

• “Top” depends on what matters to you (product engineering vs. outsourced services vs. captive R&D, company size, domain expertise, on‑shore support, price). The firms above are commonly cited as leading Sri Lankan engineering organizations and delivery centers; I included primary sources for each.
• If you want, I can: (a) narrow this to firms that specialize in a specific technology or domain (cloud, fintech, trading systems, mobile/product engineering), (b) produce contact/office links, or (c) rank these by size, revenue, or engineering headcount (I’ll need to look up the latest numbers). Which would you prefer?